1. Field of the Invention
The present invention relates to an optical interference measuring apparatus.
2. Description of the Related Art
Previously, an optical interference measuring apparatus which measures an optical path difference based on a phase difference between a test signal and a reference signal has been proposed. For example, Japanese Patent No. 3739987 discloses an optical interference measuring apparatus which measures the optical path difference between a test optical path and a reference optical path using an interference of two multiple-wavelength light sources (optical frequency comb sources) whose frequency intervals and center frequencies are different from each other. Japanese Patent Laid-open No. 2009-25245 discloses an optical interference measuring apparatus which calculates a distance based on a phase difference between a measurement signal and a reference signal for every frequency component.
Ki-Nam Joo & Seung-Woo Kim (2006) “Absolute distance measurement by dispersive interferometry using a femtosecond pulse laser “OPTICS EXPRESS, Vol. 14, No 13, pp. 5954-5960 discloses a method of dividing an interference signal of a test optical path and a reference optical path generated from one multiple-wavelength light source by a diffracting grating. In the method disclosed in the literature, an interference signal corresponding to each of the spectra of the multiple-wavelength light source is separated to perform a homodyne measurement of a phase change caused by an optical path difference with respect to frequencies. Japanese Patent Laid-open No. 2009-25245 discloses, as a method of dividing the interference signal of the test optical path and the reference optical path by one multiple-wavelength light source by a diffraction grating, a method of inserting a frequency shifter in one of the test optical path and the reference optical path to perform heterodyne detection.
However, the conventional optical interference measuring apparatus which uses two multiple-wavelength light sources whose frequency intervals and center frequencies are different from each other has two problems as follows. First, because the measurement accuracy of the optical interference measuring apparatus is sensitive to frequency stability of the two multiple-wavelength light sources, particularly to a difference of frequency intervals of the multiple-wavelength light sources, improving the accuracy of the optical interference measuring apparatus is difficult. Second, in a conventional optical interference measuring apparatus, because two frequency comb generators are necessary and also a complicated reference oscillator unit for stabilizing a difference of the frequency intervals between the multiple-wavelength light sources with high accuracy is necessary, the optical interference measuring apparatus is expensive.
Particularly, in an optical interference measuring apparatus using one multiple-wavelength light source, because a phase detection accuracy of an interference signal for homodyne detection is low, highly accurate distance measurement is difficult. Even if the frequency shifter is inserted in one of the test optical path and the reference optical path to perform the heterodyne detection, because the optical path difference between the test optical path and the reference optical path is enlarged by inserting the frequency shifter, improving the accuracy of the apparatus is difficult due to the influence of fluctuations or the like. Further, when considering a multiaxial application where a plurality of interferometers are provided to one light source, the apparatus is expensive because the frequency shifter is necessary for every interferometer.